Oil and gas production in deep and ultra-deep water presents many challenges, one of them being the design of technical and cost effective riser systems (the conduit between the sea floor and the host platform). In many deepwater areas where hydrocarbons are found, such as the Gulf of Mexico, severe current loading is invariably expected. High current can generate vortex-induced vibrations (VIV) that give rise to high rates of riser fatigue damage accumulation. As water depth increases, riser designs become more varied and VIV behavior presents one of the biggest uncertainties facing the riser engineers.
A major concern in offshore oil and gas operations, therefore, is uncertainty as to how much life remains in the riser systems, whether a drilling or production riser. Miscalculations as to remaining life can lead to sudden and catastrophic losses in containment of hydrocarbons. As such, exploration and production companies are more likely to err on the side of conservatism, for example, choosing to shut-in production with million dollar repercussions in revenue, rather than risk failure.
At present, the stress and strains in a steel catenary production riser (SCR) are not monitored, but instead are estimated based on sea current data, theoretical models, estimates of boundary conditions, and changeable structural data. Confidence in the calculations is low and a factor of safety of ten to twenty is applied to the calculated life. Judgment and guesswork are used when predicting whether an existing SCR's production life should be extended. Misjudgment in the remaining life of a riser could lead to catastrophic loss of containment of hydrocarbons and the resulting negative impacts would be severe.
Similarly, the fatigue effect of large metocean events on risers is not well known. Metocean events may include extreme wind speeds or storm surges from hurricanes and large eddy currents at great depths. The fatigue of any riser that has experienced these events introduces an additional level of uncertainty. By monitoring the riser through one of these large metocean events, the precise level of fatigue will be recorded and evaluated. This data could also allow better assessment of previous fatigue due to large metocean events.
In addition, the soil/pipe interaction of a SCR at the Touch Down Point (TDP), the point where the riser contacts the sub-sea floor, is not well understood. This point is where the greatest changes in stress and strain exist on the SCR. Strain monitoring at the TDP would improve the understanding of this interaction. Once a better understanding is gained, improvements in design and decision-making can be made.
Additionally, operational moves by the platform supporting the SCR can cause significant movement of the TDP adding to suspected trenching and interaction of the pipe with the sea floor. Large trenches have been observed in SCR-pipeline surveys, leading to concerns as to the impact on the serviceability of the riser. Optical strain monitoring would significantly address this uncertainty and allow for operational guidance in moving the platform around. Likewise, monitoring of the top-end of the SCR will assist in guiding operational platform movements, by prescribing and monitoring acceptable stress and deformation (inclination) of the top end of the SCR.
Large temperatures encountered in many reservoirs produce temperatures in excess of 2000° F. (often as high as 350° F.) in the riser pipe as the hydrocarbons move up to the surface. Temperatures of this magnitude can cause very large mechanical strains and cycling of strains as the temperatures fluctuate. This is poorly understood through present theoretical models, and is of great concern in the safe operation of production-type risers and flowlines on the sea floor.
To aggravate conditions, hydrocarbons can often be “sour” in that they produce highly corrosive environments on the pipe interior. Whereas some strategies to counter this include very expensive corrosion resistant alloys (CRA's), monitoring of changes in the wall thickness would be of great importance in safe offshore operations.
Accordingly, there remains a substantial need for a solution to the problem of monitoring fatigue, operational behavior, and stresses on drilling and production risers.